The present invention relates to an improved bilayer photoresist and process for its use in lithography for manufacturing semiconductor devices.
There is a desire in the industry for higher circuit density in microelectronic devices made using lithographic techniques. One method of achieving higher area density is to improve the resolution of circuit patterns in resist films. It is known in the art that increasing the numerical aperture (NA) of the lens system of the lithographic imaging tool increases the resolution at a given wavelength. However, increasing the NA results in a decrease in the depth of focus (DOF) of the imaging radiation, thereby requiring a reduction in the thickness of the imaging resist film. Further, the industry-wide shift to shorter wavelength exposure systems also results in a decrease in the DOF. A decrease in the resist film thickness can lead to problems in subsequent processing steps (e.g., ion implantation and etching).
In order to overcome these problems, bilayer resists have been developed. Bilayer resists generally comprise a top thin film imaging layer coated on a thick organic underlayer. The resist is patterned by: (i) imagewise exposure and development of the top layer, and then (ii) anisotropically transferring the developed pattern in the top layer through the thick underlayer to the substrate. Suitably, the top layer contains precursors to refractory oxides such as silicon, boron, or germanium which enable the use of oxygen-reactive ion etching (RIE) in the image transfer step. However, the incorporation of silicon into the photoresist film often leads to the degradation of resolution and imaging performance.
Bilayer resists are known in the art. However, these resists were generally developed before the advent of deep U.V. lithography (e.g., 248 nm and 193 nm) and are of little utility for high-resolution imaging. For example, in the review article xe2x80x9cPolymeric Silicon-containing Resist Materialsxe2x80x9d, Advanced Material for Optics and Electronics, Vol. 4, pp. 95-127 (1994), there is disclosed on page 112 a positive bilayer resist having a top layer comprising the copolymer poly(co-trimethylsilylmethyl methacrylate and monooximido xcex1 diketone). The top layer is imaged by radiation chain scission and the image is transferred with oxygen R.I.E. However, the resist is not commercially viable due to slow photospeed and other resist performance problems. Therefore, there still is a need in the art for a bilayer photoresist suitable for commercial use.
It is therefore an object of the present invention to provide an improved bilayer photoresist.
Other objects and advantages will become apparent from the following disclosure.
The present invention relates to a process for forming bilayer resist images on a substrate with a chemically-amplified, radiation-sensitive bilayer resist. The bilayer resist is disposed on a substrate and comprises (i) a top imaging layer comprising a radiation-sensitive acid generator and a vinyl polymer or copolymer formed by the polymerization of monomers, including one or more monomers selected from acrylate, methacrylate, hydroxystyrene (optionally substituted with C1-6 alkyl), and C5-20 cyclic olefin monomers, where preferably the polymer has an acid-cleavable silylethoxy group; and (ii) an organic underlayer. The present invention relates to the process for using the bilayer resist to make resist images in a film in the manufacture of integrated circuits.
A more thorough disclosure of the present invention is presented in the detailed description which follows.
The present invention relates to a positive tone, chemically-amplified, radiation-sensitive bilayer resist. The bilayer resist comprises (a) a top imaging layer comprising (i) a radiation-sensitive acid generator; (ii) a vinyl polymer or copolymer formed by the polymerization of one or more monomers, including a monomer selected from acrylate, methacrylate, hydroxystyrene (optionally substituted with C1-6 alkyl), and C5-20 cyclic olefin monomers (preferably C7-15, e.g., norbornene and tetracyclododecane); and (iii) a compound having a silylethoxy acid-cleavable group; and (b) a polymeric organic underlayer. The ethoxy portion of the silylethoxy group is optionally substituted with C1-6 alkyl, phenyl, or benzyl. The vinyl polymer may optionally comprises other types of monomers known to those skilled in the art. Preferably, the silicon-containing, acid-cleavable group is bonded to the vinyl polymer.
The resist is chemically amplified in that the proton produced in the photoreaction of the radiation-sensitive acid generator initiates catalytic cleavage reactions of the acid-cleavable group independent of the radiation, thereby increasing the effective quantum yield to values above 1.
The silicon-containing, acid-cleavable group consists of silicon atoms, carbon atoms, hydrogen atoms, and one oxygen atom. Suitable acid-cleavable silylethoxy groups have the formula R1 R2 R3 Si (CRxe2x80x22)2O, where each Rxe2x80x2 is independently hydrido, C1-6 alkyl (e.g., methyl), phenyl, or benzyl optionally substituted with C1-6 alkyl and R1, R2, and R3 are each independently hydrido, alkyl preferably lower (C1-6) alkyl or (R4)3 Si, where each R4is independently hydrido or lower alkyl. Preferred silicon-containing, acid-cleavable groups are C1-6 alkyl silylethoxy; mono, bis, tris (C1-6 alkyl silyl) silylethoxy. The bridging alkylene (CR2xe2x80x2)2 group is important in that it enables nonhydrolytic, solid state, acid-catalyzed cleavable of the acid-cleavable group which is believed to occur through the formation of a beta silyl carbocation as a cleaving group. The top imaging layer of the present invention is not crosslinked (uncrosslinked) and has a high silicon content to give enhanced stability against reactive ion etching. The top imaging layer is also hydrolytically stable and the top layer composition has enhanced shelf stability.
In one embodiment of the present invention, the top imaging layer comprises a radiation-sensitive acid generator and an acrylate or methacrylate polymer having an acid-cleavable, silicon-containing group (e.g., silylethoxy) attached to the carbonyl of the acrylate or methacrylate.
The silicon-containing acrylate or methacrylate can be used as a homopolymer or can be a copolymer. Suitable comonomers include (i) acrylate or methacrylate monomers with lower alkyl ester groups, (ii) acrylic acid or methacrylic acid monomers, (iii) methacrylate or acrylate monomers with other types of acid labile ester groups such as tertiary alkyl esters (t-butyl esters), or (iv) hydroxystyrene.
In an alternative embodiment, the polymer in the top imaging layer can be an alicyclic polymer having an alicyclic backbone (e.g., formed from cyclic olefin monomer) where the silicon-containing, acid-cleavable group (e.g., silylethoxy) is preferably bonded to a carbonyl group attached to the cycloalkyl ring. Suitable monomers include functionalized norbornene and tetracyclododecane.
In another alternative embodiment, the top imaging layer comprises a vinyl polymer, an acid generator, and a compound having a silicon-containing, acid-cleavable group. Suitable compounds are bisphenol A and steroids (e.g., substituted androstane as disclosed in Allen et al., U.S. Pat. No. 5,580,694, issued Dec. 3, 1996, the disclosure of which is incorporated herein by reference for all purposes). Other suitable compounds will be known to those skilled in the art.
In another alternative embodiment, the polymer in the top imaging layer is polyhydroxystyrene where the silicon-containing, acid-cleavable group (e.g., silylethoxy) is bonded directly to the aromatic ring (e.g., as a protected hydroxy substituent).
The second component of the top imaging layer is the radiation-sensitive acid generator. Upon exposure to radiation, the radiation-sensitive acid generator generates an acid. Suitable acid generators include triflates (e.g., triphenylsulfonium triflate or bis-(t-butyl phenyl) iodonium triflate), pyrogallol (e.g., trimesylate of pyrogallol), onium salts such a triarylsulfonium and diaryl iodonium hexafluoroantimonates, hexafluoroarsenates, trifluoromethane sulfonates and others; iodonium sulfonates and trifluoromethanesulfonate esters of hydroxyimides, alpha-alphaxe2x80x2-bis-sulfonyl diazomethanes, sulfonate esters of nitro-substituted benzyl alcohols and napthoquinone-4-diazides and alkyl disulfones. Other suitable photoacid generators are disclosed in Allen""s U.S. Pat. Nos. 5,045,431 and 5,071,730, and Reichmanis et al.""s review article (Chemistry of Materials, Vol. 3, page 395 (1991)), the disclosures of which are incorporated herein by reference for all purposes.
The two-component top imaging layer generally comprises about 1 to 10 weight % of the acid generator and about 90 to 99 weight % of the polymer. The top imaging layer may optionally comprise other minor components such as dissolution inhibitors, coating enhancers, surfactants, bases, and other compounds known to those skilled in the art.
Suitable organic, polymeric, planarizing underlayers for the resist of the present invention include hard-baked diazonaphthoquinone (DNQ) novolac, polyimides, polyesters, polyacrylates and the like. DNQ novolac is the preferred polymer for the underlayer. Other crosslinkable polymers known to those skilled in the art can also be used as the underlayer.
The present invention relates to a process for generating a positive bilayer resist image on a substrate comprising the steps of: (a) coating a substrate with an organic underlayer; (b) coating the organic underlayer with a top layer comprising a radiation-sensitive acid generator and a vinyl polymer having a silicon-containing, acid-cleavable group; (c) imagewise exposing the top layer to radiation; (d) developing the image in the top layer; and (e) transferring the image through the organic underlayer to the substrate.
The first step of the process of the present invention involves coating the substrate with a layer comprising an organic polymer dissolved in a suitable solvent. Suitable substrates are comprised of silicon. Suitably, the surface of the substrate is cleaned by standard procedures before the layer is disposed thereon. Suitable solvents for the organic polymer underlayer include propylene glycol methyl ether acetate. The layer can be coated on the substrate using art-known techniques such as spin or spray coating, or doctor blading. The layer is then heated to an elevated temperature of about 100-250xc2x0 C. for a short period of time of about 1-30 minutes to drive off solvent and optionally thermally induce crosslinking. The dried underlayer layer has a thickness of about 0.5-20 microns, preferably about 1 micron.
In the second step of the process, the components of the top imaging layer are dissolved in a suitable solvent such as propylene glycol methyl ether acetate (AMGA) and coated onto the underlayer of organic polymer. It is desired that the imaging layer not admix with the underlayer layer during the coating process. The top layer has a thickness of about 0.1 to 0.3 microns.
In the next step of the process, the film stack (the top layer and underlayer) is imagewise exposed to radiation, suitably electromagnetic radiation or electron beam radiation, preferably ultraviolet radiation suitably at a wavelength of about 190-365 nm (193/248/254/365/x-rayxe2x80x94hard and soft, e.g., euv 13 nm), preferably 193 or 248 nm. Suitable radiation sources include mercury, mercury/xenon, and xenon lamps. The preferred radiation source is ArF excimer or KrF excimer. At longer wavelengths (e.g., 365 nm) a sensitizer may be added to the top imaging layer to enhance absorption of the radiation. Conveniently, due to the enhanced radiation sensitivity of the top layer of the resist film, the top layer of the film has a fast photospeed and is fully exposed with less than about 100 mJ/cm2 of radiation, more preferably less than about 50 mJ/cm2. The radiation is absorbed by the radiation-sensitive acid generator or sensitizing agent to generate free acid which causes cleavable of the silicon-containing, acid-cleavable group and formation of the corresponding carboxylic acid or phenol.
Preferably, after the film has been exposed to radiation, the film is again heated to an elevated temperature of about 90-120xc2x0 C. for a short period of time of about 1 minute.
The next step involves development of an image in the top layer with a suitable solvent. Suitable solvents for development of a high contrast, positive image include an aqueous base, preferably an aqueous base without metal ions such as tetramethyl ammonium hydroxide or choline. The development results in removal of the exposed areas of the top layer of the film.
The last step of the process involves transferring of the developed image in the top layer through the underlayer to the substrate by known techniques. Preferably, the image is transferred by etching with reactive ions such as plasma etching and reactive ion etching. Suitable plasma tools include electron cyclotron resonance (ECR), helicon, inductively coupled plasma (ICP), and transmission-coupled plasma (TCP) systems. Suitably, oxygen-reactive ion etching (magnetically enhanced) is utilized to transfer the image through the underlayer. Etching techniques are well known in the art and equipment is commercially available to etch films. The developed film has high aspect ratio, high etch resistance, enhanced resolution, and straight wall profiles.
The bilayer resist of the present invention may be used to make an integrated circuit assembly, such as an integrated circuit chip, multichip module, circuit board, or thin film magnetic heads. The integrated circuit assembly comprises a circuit formed on a substrate by using the process of the present invention, and then additionally forming a circuit in the developed film on the substrate by art-known techniques. After the substrate has been exposed, circuit patterns can be formed in the exposed areas by coating the substrate with a conductive material such as conductive metals by art-known dry-etching techniques such as evaporation, sputtering, plating, chemical vapor deposition, or laser-induced deposition. The surface of the film can be milled to remove any excess conductive material. Dielectric materials may also be deposited by similar means during the process of making circuits. Inorganic ions such as boron, phosphorous, or arsenic can be implanted in the substrate in the process for making p or n doped circuit transistors. Other means for forming circuits are well known to those skilled in the art.